Method of reducing the cleaning requirements of a dielectric chuck surface

ABSTRACT

The present invention pertains to an apparatus and method useful in semiconductor processing. The apparatus and method can be used to provide a seal which enables a first portion of a semiconductor processing chamber to be operated at a first pressure while a second portion of the semiconductor processing chamber is operated at a second, different pressure. 
     The sealing apparatus and method enable processing of a semiconductor substrate under a partial vacuum which renders conductive/convective heat transfer impractical, while at least a portion of the substrate support platform is under a pressure adequate to permit heat transfer using a conductive/convective heat transfer means. The sealing apparatus comprises a thin, metal-comprising layer, typically in the form of a strip or band, brazed to at least two different surfaces within said processing chamber, whereby the first and second portions of the semiconductor processing chamber are pressure isolated from each other. Preferably, the metal-comprising layer exhibits a cross-sectional thickness of less than about 0.039 in. (1 mm). 
     The invention is particularly useful when there is a differential in linear expansion coefficient of at least 3×10 −3  in./in./° C., measured at 600° C., between the surfaces to be bridged by the thin, metal-comprising layer.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional application of application Ser. No.08/709,136, filed Sep. 6, 1996, now U.S. Pat. No. 5,673,167, which is acontinuation of application Ser. No. 08/436,058, filed May 5, 1995,abandoned, which is a divisional application of application Ser. No.08/073,029, filed Jun. 7, 1993, now U.S. Pat. No. 5,511,799.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus and method which can beused to provide a seal between two portions of a semiconductorprocessing reactor which are operated at different pressures. Theapparatus and method are particularly useful when the processing reactoroperates over a broad temperature range (about 600° C.) and the sealmust bridge two materials having a substantially different coefficientof expansion.

2. Description of the Background Art

In the fabrication of electronic components such as semiconductordevices, the manufacturing process frequently requires that a substratebe cooled while that substrate is exposed to a partial pressure of about10⁻³ Torr or lower. Processes which require substrate cooling under suchpartial vacuum conditions include, for example, physical vapordeposition (PVD), ion injection and particular forms of plasma etching.

PVD is used to deposit a thin film on a substrate. Films of materialssuch as, for example, aluminum, titanium, tungsten, tantalum, tantalumnitride, cobalt, and silica may be deposited on ceramic, glass orsilicon-derived substrates using PVD processes such as a sputteringprocess. In a typical sputtering process, a low pressure atmosphere ofan ionizable gas such as argon or helium is produced in a vacuumchamber. The pressure in the vacuum chamber is reduced to about 10⁻⁶ to10⁻¹⁰ Torr, after which argon, for example, is introduced to produce anargon partial pressure ranging between about 0.0001 Torr (0.1 mTorr) andabout 0.020 Torr (20 mTorr). Two electrodes, a cathode and an anode, aregenerally disposed in the vacuum chamber. The cathode is typically madeof the material to be deposited or sputtered, and the anode is generallyformed by the enclosure (particular walls of the vacuum chamber, or theplatform upon which the substrate sits, for example). At times, anauxiliary anode may be used or the article to be coated may serve as theanode. A high voltage is applied between these two electrodes, and thesubstrate to be coated is disposed upon a platform positioned oppositethe cathode. The platform upon which the substrate sets is often heatedand/or cooled, and heat is transferred between the platform and thesubstrate, to assist in obtaining a smooth, even thin film coating uponthe substrate. To obtain a smooth, even film coating, it is desirable tomaintain the substrate at a uniform temperature within a few ° C.;preferably, the temperature is near but below the melting point of thematerial from which the film is being formed. It is very important thatthe substrate temperature be repeatable each time a given process iscarried out. Thus, the heat transfer between the platform and thesubstrate must be uniform and repeatable.

When the pressure in the process (vacuum) chamber is about 1 Torr orless, convective/conductive heat transfer becomes impractical. This lowpressure environment affects heat transfer between the substrate and thesupport platform; it also affects heat transfer between the supportplatform and heat transfer means such as a heating or cooling coil usedadjacent to the platform to heat or cool the platform.

Since the substrate and the platform typically do not have the perfectlylevel surfaces which would enable sufficiently even heat transfer bydirect conduction, it is helpful to provide a heat transfer fluidbetween the platform and the substrate, to assist in providing even heattransfer between the support platform and the substrate. It is known inthe art to use one of the gases present in a PVD sputtering process as aheat transfer fluid between the support platform and the substrate. Thefluid is typically a gas such as helium, argon, hydrogen, carbontetrafluoride, or hexafluoroethane, for example, or other suitable gasthat is a good heat conductor at low pressure. The fluid is generallyapplied through multiple openings or into exposed channels in thesubstrate-facing surface of the support platform. Presence of the heattransfer fluid between the substrate and support platform surfaceestablishes a nearly static gas pressure which commonly ranges fromabout 1 Torr to about 100 Torr, depending on the particular filmdeposition process.

The positive pressure created by the heat transfer fluid used betweenthe back (non-processed) side of the substrate and its support platform,as described above, tends to bow a thin substrate which is mechanicallyclamped at its edge. Bowing of the substrate in excess, for example 10micrometers at the center of a typical silicon wafer substrate, wasobserved when the periphery of the substrate was held by mechanicalclamps. This substrate bowing reduces the amount of heat transfer nearthe center of the substrate and results in uneven heating of thesubstrate in general. Further, the gas used to provide a heat transferfluid between the substrate and the support platform leaks from theedges of the substrate so that a constant net flow of fluid occurs frombeneath the substrate into the process vacuum chamber. The amount of gasleakage commonly ranges from about 10% to about 30% of the gas usedduring processing of the substrate.

One means of avoiding bowing of the substrate and of reducing heattransfer fluid leakage at the substrate edge is the use of anelectrostatic chuck as the support platform. An electrostatic chucksecures the entire lower surface of a substrate by Coulombic force andprovides an alternative to mechanical clamping of the substrate to thesupport platform. When a substrate is secured to the platform using anelectrostatic chuck, the flatness of the substrate during processing isimproved. In a typical electrostatic chuck, the substrate (comprised ofa semiconductor material or a non-magnetic, electrically conductivematerial) effectively forms a first plate of a parallel-plate capacitor.The remainder of the capacitor is generally formed by the substratesupport platform which typically comprises a dielectric layer positionedon the upper surface of a second conductive plate.

The support platform upon which the substrate sets can be heated orcooled in a variety of manners, depending on the kind of process to becarried out in the process chamber. For example, radiant, inductive, orresistance heating can be used to heat a support platform; the platformcan be heated or cooled using a heat transfer fluid which is circulatedthrough internal passageways within the support platform; in thealternative, heating or cooling can be achieved using a heat transfersurface such as a heating or cooling coil which is located adjacent to,frequently in contact with, the support platform. The means of heatingor cooling the support platform often depends on the materials ofconstruction of the platform itself. When the partial pressure in theprocess chamber is less than about 1 Torr, the support platform cannotbe heated or cooled using a heat transfer surface adjacent the supportplatform, since convection/conduction of heat cannot occur at apractical rate.

There are numerous materials of construction and structuralpossibilities described in the art which can be used to form anelectrostatic chuck. The means for heating and cooling the electrostaticchuck (from which the substrate can be heated or cooled) depends on thematerials of construction used and the overall structure of theelectrostatic chuck.

U.S. Pat. No. 4,771,730 to Masashi Tezuka, issued Sep. 20, 1988describes an electrostatic chuck comprised of a conductive specimentable (probably constructed from a metallic material such as aluminum)having hollow conduits therein for the circulation of water. Mounted tothe upper surface of the specimen table is an electrode comprising anelectrode plate constructed of a metal such as aluminum, surrounded by adielectric film of a material such as Al₂O₃ (alumina). The substrate tobe processed sets upon the upper surface of the dielectric filmmaterial. It is readily apparent that the sandwiched materials ofconstruction which make up the electrostatic chuck have vastly differentcoefficients of expansion. Upon exposure of the sandwiched materials tooperational temperature ranges of several hundred degrees Centigrade,stress is created between the sandwiched layers which can lead todeformation or fracture of the more fragile layers and to poorperformance and eventual failure of the electrostatic chuck in general.

U.S. Pat. No. 5,155,652 to Logan et al., issued Oct. 13, 1992, disclosesan electrostatic chuck assembly including, from top to bottom: a topisolation layer; an electrostatic pattern layer comprised of anelectrically conductive electrostatic pattern disposed on a substrate; aheating layer comprised of an electrically conductive heating patterndisposed on a substrate; a support layer; and a heat sink base havingbackside cooling and insulating channels provided therein. The preferredmaterial for the isolation layer is Boralloy 11®, a pyrolytic boronnitride available from Union Carbide. The electrostatic pattern layer isnot particularly defined. The heating layer is comprised of a substrate,preferably pyrolytic boron nitride having a conductive heating patterndisposed thereon. The conductive heating pattern is preferably comprisedof a pyrolytic graphite. The support layer is preferably comprised of aboron nitride having metal vias disposed therethrough for conductingelectrical energy to metal vias within the heating layer. The heat sinkbase is comprised of a thermally conductive block of material havingclearance holes extending therethrough for facilitating electricalcontact with the metal vias of the support layer. The heat sink basealso has channels therein for the circulation of a cooling fluid. Thematerial of selection for heat sink base is said to be critical becauseit must match the thermal expansion rate of all the other layers in thestructure. It is recommended that KOVAR®, an iron/nickel/cobalt alloyavailable from Westinghouse Electric Co. be used to form the heat sinkbase.

The heat sink base is said to be bonded to the bottom of the supportlayer using one of several techniques. The techniques include brazing,whereby gold contact pads are deposited on the respective bondingsurfaces, the pieces are fitted together, and the assembly is heated ina brazing furnace. A second method of bonding the heat sink base to thebottom of the support layer is to apply a thermally conductive ceramiccement. A third method is to mechanically clamp the two pieces togetherby fabricating a flange on the bottom of the support layer and a clampring on the top of the heat sink base.

The kind of structure described in the Logan et al. is very expensive toconstruct, both in terms of materials of construction and fabrication ofthe structure itself.

U.S. Pat. No. 5,191,506 to Logan et al., issued Mar. 2, 1993 describesan electrostatic chuck assembly including, from top to bottom: a topmultilayer ceramic insulating layer; an electrostatic pattern layerhaving a conductive electrostatic pattern disposed upon a multilayerceramic substrate; a multilayer ceramic support layer; and a heat sinkbase having backside cooling channels machined therein. The multilayerceramic structures are bonded together using known techniques applicableto multilayer ceramics, and the heatsink base is brazed to the bottom ofthe multilayer ceramic support layer. The materials of construction areessentially equivalent to the materials of construction described inU.S. Pat. No. 5,155,652. The heat sink base is said to be brazed to theelectrostatic pattern layer by depositing gold contact pads on therespective bonding surfaces, fitting the pieces together, and heatingthe assembly in a brazing furnace.

In the above-referenced U.S. patents, when cooling is involved, thecooling is accomplished by using a fluid flowing: a) in direct contactwith either a specialized metal alloy having a coefficient of expansionmatched to that of a dielectric material with which it was in contact,or b) in direct contact with the dielectric material itself. This isnecessary since the entire vacuum chamber in which substrate processingis carried out is operated at a partial vacuum which rendersconductive/convective heat transfer impractical. The use a specializedmetal alloy heatsink base of the kind described in the above-referencedpatents is very expensive. Further, direct contact of the cooling fluidwith the kinds of dielectric materials generally described may notprovide effective heat transfer and may result in fracture of thedielectric material itself. The ceramic materials typically used asdielectrics are commonly sensitive to temperature differential, and thetemperature differential between a heating element and a nearby coolingelement can cause the ceramic to fracture.

It would be very advantageous to have a means for cooling anelectrostatic chuck (or any other substrate support platform used in lowpressure semiconductor processing) which did not require the use of suchexpensive materials of construction and which provided an efficientmeans of heat transfer.

SUMMARY OF THE INVENTION

In accordance with the present invention, an apparatus and method aredisclosed which can be used to provide a seal between two portions of asemiconductor processing reactor which are operated at differentpressures. The seal enables processing of the substrate under a partialvacuum which renders conductive/convective heat transfer impractical,while at least a portion of the substrate support platform, removed fromthe substrate contacting portion, is under a pressure adequate to permitheat transfer using a conductive/convective heat transfer means.

The apparatus of the present invention comprises a sealing apparatuscapable of withstanding a pressure differential, typically about 15 psi(15.15×10⁶ dynes/cm²) or less, over a temperature range of at least 300°C., while bridging at least two materials having a substantialdifference in linear expansion coefficient. The difference in linearexpansion coefficient depends upon the composition of the materialsbeing bridged, but is typically at least about 3×10⁻³ in./in./° C.(m/m/° C.), measured at about 600° C. Preferably, the sealing means canwithstand a pressure differential of at least about 15 psi duringoperational temperatures ranging from about 0° C. to about 600° C. whilebridging two materials having a difference in linear expansioncoefficient ranging from about 3×10⁻³° C.⁻¹ to about 25×10⁻³° C.⁻¹,measured at 600° C.

In particular, the sealing means comprises a thin metal-comprising layerwhich is coupled to each of the materials which the seal bridges. Thematerial comprising the thin metal-comprising layer preferably exhibitsa coefficient of expansion similar to one of the materials to bebridged. Typically the metal-comprising layer is selected to have athermal expansion coefficient relatively close to the lowest thermalexpansion coefficient material to be bridged. When the metal-comprisinglayer bridges between a metal (or metal alloy) and a ceramic materialsuch as alumina or aluminum nitride, the metal-comprising layer isselected to have a linear thermal expansion coefficient in the range ofabout 2×10⁻³ to about 6×10⁻³ in./in./° C. at about 600° C., andpreferably in the range of about 2×10⁻³ to about 4×10⁻³ in./in./° C. atabout 600° C. The cross-sectional thickness of the thin metal-comprisinglayer is typically less than 0.039 in. (1 mm).

The preferred method of coupling the thin metal-comprising layer to thematerials being bridged is brazing. By brazing, it is meant that thethin, metal-comprising layer is coupled to each of the materials beingbridged using a third material as a coupling agent. The coupling agentor brazing material need not have a linear thermal expansion coefficientas low as that of the thin, metal-comprising layer, since the brazingmaterial is designed to operate in combination with the relativelyflexible, thin, metal-comprising layer. A critical parameter for thebrazing material is that it wet out the surface of and bond well to thethin, metal comprising layer and to the surface of each material to bebridged. Another critical parameter is that the brazing material havethe capability of relaxing stresses (that it is a low stress material).Thus, a material having a low Young's modulus with a high yield pointmakes a desirable brazing material.

The method of the present invention, is useful in a semiconductorprocessing chamber or reactor when it is necessary to provide a sealbetween processing areas within the chamber, when the seal bridges atleast two surfaces which exhibit different linear thermal expansioncoefficients, and when the operational temperature range for thesemiconductor processing chamber is at least 300° C., the methodcomprises the steps:

a) providing at least two material surfaces which exhibit differentlinear thermal expansion coefficients;

b) providing a thin, metal-comprising layer of material having a linearcoefficient of expansion closer to the lowest linear thermal coefficientof expansion material to be bridged; and

c) brazing the metal comprising layer of material to each of the atleast two surfaces which must be bridged by the seal, whereby themetal-comprising layer, brazing material and surfaces to which themetal-comprising layer is brazed act as a sealing apparatus.

Preferably the metal-comprising layer of material is braised to eachsurface to be bridged using a technique which provides an intermixingbetween molecules from the metal-comprising layer of material and themolecules of the material comprising each surface to be bridged. Thepreferred cross-sectional thickness of the metal-comprising layer ofmaterial is about 0.39 in. (1 mm) or less. Typically, the difference inlinear coefficient of expansion of the materials comprising at least twoof the surfaces to be bridged is at least about 3×10⁻³ in./in./° C.(m/m/° C.) at about 600° C.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a schematic of a substrate support platform and auxiliaryapparatus used in semiconductor processing which employs the presentinvention sealing means.

FIG. 2 illustrates the present invention in one preferred embodiment;the same embodiment as shown in FIG. 1.

FIG. 3 shown a second preferred embodiment of the present invention.

FIG. 4 illustrates the preferred embodiment shown in FIG. 1, includingdimensional indicators.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention pertains to a sealing means which can be used tobridge between two materials having substantially different linearcoefficients of expansion during semiconductor processing, wherebydifferent portions of the semiconductor processing chamber can beoperated at different absolute pressures while the process operates overa large temperature range, for example, between room temperature andabout 600° C.

One skilled in the art, having read the present disclosure, will havenumerous applications for the present invention. However, the inventionis especially useful in semiconductor processing when the substratebeing processed is subjected to sputtering, ion injection or etchingprocesses which generate heat, when an electrostatic chuck is used tosupport the substrate; and, when it is necessary to be able to cool theelectrostatic chuck. The preferred embodiment of the invention describedbelow is directed toward a sputtering process, but is equally applicableto any process which causes the substrate to heat up, requiring a heatremoval capability.

FIG. 1 shows an apparatus 100 for use in semiconductor processing,including a substrate support platform 102. Substrate support platform102 functions as an electrostatic chuck, wherein a non-magnetic,semiconductor or conductive substrate 104 forms the first plate of acapacitor; a dielectric inner layer is furnished by a portion of upperplaten 106; and a second conductive plate is furnished by conductivelayer 111 embedded within upper platen 106.

Support platform upper platen 106, is fabricated from a high thermalconductivity dielectric material such as pyrolytic boron nitride,aluminum nitride, silicon nitride, alumina or an equivalent material.Platform housing 108 is typically fabricated from a material such asstainless steel or aluminum, for example, with stainless steel beingpreferred.

Preferably upper platform platen 106 contains an embedded, electricallyconductive heating pattern 110 which can be used to heat upper platen106. Optionally, substrate support platform 102 comprises a removableinsert 112 and may comprise a shadow ring 114. Insert 112 is arecyclable element which is used to capture back-scattered depositionmaterials and to prevent the need to clean upper platen 106. Insert 112is constructed from a relatively inexpensive material having acoefficient of expansion similar to that of the dielectric materialcomprising upper platen 106. An example of an acceptable insert materialis alumina.

Lift fingers 116 are used in combination with substrate support platform102 to enable the lifting of a substrate 104 above upper platen 106, sosubstrate 104 can be grasped by a mechanical device (not shown) usedmove the substrate within the semiconductor processing chamber (notshown) which surrounds support platform 102.

Cooling of substrate 104 is accomplished by cooling upper platen 106 andtransferring heat from substrate 104 to upper platen 106. Upper platen106 is cooled using a cooling coil 118, wherein a first heat transferfluid passes into cooling coil 118 at entry 120 and exits through exit122. Although cooling coil 118 can be used directly adjacent platformplaten 106, typically a cooling plate 124 is used in combination withcooling coil 118 to provide a more uniform heat transfer between upperplaten 106 and cooling coil 118. Use of cooling plate 124 reduces theamount of temperature differential required at an individual location onthe surface of upper platen 106. If cooling coil 118 is located rightnext to the dielectric material comprising upper platen 106, an abrupttemperature difference at this location can cause damage to thestructure of the dielectric material.

Spring support 126 is used to maintain close contact between upperplaten 106 and cooling plate 124. Cooling coil 118 is typicallyfabricated from aluminum, stainless steel, or copper, for example.Cooling plate 124 is typically fabricated from stainless steel,tungsten, molybdenum, or Kovar®, for example. In an alternativeembodiment of cooling plate 124, a layer of material having a low linearthermal expansion coefficient, such as tungsten, molybdenum, or Kovar®,could be braised to upper platen 106, with a second heat conductiveplate of a material such as stainless steel being attached to thatlayer.

As described in the Background Art section of this disclosure, thesemiconductor processing of substrate 104 is carried out in a partialvacuum, wherein the absolute pressure is frequently as low as 0.1 mTorr.Thus, a second heat transfer fluid may be used to obtain heat transferbetween substrate 104 and upper platen 106 (despite the relatively flatsurfaces and good contact between substrate 104 and upper platen 106).Conduit 128 is used to transfer the second heat transfer fluid from asource not shown in FIG. 1. Conduit 128 is attached to platform platen106, whereby the second heat transfer fluid can flow to opening 130 andinto open channels 132 upon the upper surface 134 of support platen 106.A thermocouple 136 is used to sense the temperature of platform platen106 and to transfer information to a controller which calls for eitherheating of platform platen 106 by electrically conductive elements 110or for cooling of platform platen 106 via cooling coil 118, which ispreferably used in combination with cooling plate 124.

Since the process chamber surrounding substrate 104 is under the partialvacuum previously described, and since this partial vacuum would preventany practical heat transfer between cooling coil 118, cooling plate 124,and platform platen 106, it is necessary to have a third heat transferfluid (in the form of a gaseous atmosphere) present between cooling coil118, cooling plate 124, and platform platen 106. This third heattransfer fluid can be one of the semiconductor process gases or air,conveniently. To enable use of the third heat transfer fluid, it isnecessary to be able to isolate the portion of the process apparatuswithin which the third heat transfer fluid is to function from theportion of the apparatus within which the substrate is treated. This isaccomplished using sealing means 138 and 140.

Sealing means 138 bridges between conduit 128 and upper platen 106.Sealing means 140 bridges between support platform housing 108 and upperplaten 106. Sealing means 138 is comprised of a thin, metal-comprisingstrip or ribbon which is braised at one edge to the surface of upperplaten 106 and at the other edge to either conduit 128 or to a firstextension 139, as shown in FIG. 1. Sealing means 140 is comprised of athin, metal-comprising strip or ribbon which is braised at one edge toupper platen 106 and at the other edge to either support platformhousing 108 or a second extension 142, as shown in FIG. 1. Sealing means138 and 140 should be capable of withstanding a pressure differential ofabout 15 psi over an operational temperature range from about 0° C. toabout 600° C.

As previously stated, upper platen 106 is preferably constructed from adielectric material such as pyrolytic boron nitride, aluminum nitride,silicon nitride, or alumina. Pyrolytic boron nitride is an anisotropicmaterial which has a thermal coefficient of expansion across its planarlength and width directions of about 1.5×10⁻³ in./in./° C. at about 600°C. and a thermal coefficient of expansion across its cross-sectionalthickness of about 40×10⁻³ in./in./° C. at about 600° C. Alumina is moreisotropic, having a thermal expansion coefficient of about 0.008×10⁻³in./in./° C. at 600° C. Aluminum nitride is also more isotropic, havinga thermal expansion coefficient of 0.01×10⁻³ in./in./° C. at 600° C.

Conduit 128 which must be attached to upper platen 106 is constructedfrom a material such as stainless steel or copper, for example; thesematerials have a linear thermal expansion coefficient at about 600° C.ranging from about 6.8×10⁻³ to 12×10⁻³ in./in./° C. at about 600° C.Support platform housing 108 is constructed from a material such asstainless steel or aluminum, for example; these materials have a linearthermal expansion coefficient ranging from about 6.8×10⁻³ to 15.9×10⁻³in./in./° C. at about 600° C. To help compensate for the effect of themismatched (difference) in linear thermal coefficient of expansionacross sealing apparatus 138 and 140 and reduce stresses created in thesealing apparatus when alumina or aluminum nitride comprises upperplaten 106, it is preferable to weld an extension 139 to conduit 128 andan extension 142 to platform housing 108, which extension is comprisedof a material having a linear thermal coefficient of expansion, in therange of about 2×10⁻³ to 4×10⁻³ in./in./° C. at about 600° C., forexample. Materials which have a linear thermal expansion coefficient inthis range and which can be used to form extensions 139 and 142 include,but are not limited to, molybdenum, tantalum, titanium, tungsten andKovar®. Since there is not a crucial amount of heat transferred betweenextension 139 and conduit 128 or between extension 140 and platformhousing 108, the extensions can be welded to conduit 128 and platformhousing 108 without creating any significant thermal expansion mismatchproblem.

FIG. 2 shows an enlargement of sealing means 140; however, the generaldescription applicable to sealing means 140 can be applied to sealingmeans 138 as well. Sealing means 140 is comprised of a thinmetal-comprised layer 202 (in the form of a strip, band or ribbon.) Thepreferred material of construction for layer 202, when upper platen 106is comprised of alumina or aluminum nitride, is one which has arelatively low linear coefficient of thermal expansion. Preferredmaterials for use in layer 202, when upper platen 106 is comprised ofpyrolytic boron nitride or a different dielectric material having ahigher thermal expansion coefficient, include materials which have ahigher thermal expansion coefficient but which tend to relax stress,such as nickel, silver, silver/titanium alloy and nickel/iron alloys,for example but not by way of limitation.

Brazing material 204, as previously described, must wet out the surfaceof and bond to the materials which are to be attached to each other bythe brazing. Further, it is important that brazing material 204 becapable of relaxing stresses created therein. Preferred materials foruse as brazing material 204 include silver, nickel, silver/coppersolder, silver/titanium solder, nickel/iron alloys and silver/titaniumalloys, by way of example. Testing to date has indicted that nickelmakes a particularly good brazing material. Testing is done byfabricating a sealing apparatus and then cycling the sealing apparatusbetween about room temperature and about 600° C. for at least severalhundred cycles. Temperature cycling is followed by vacuum testing overthe process operational temperature range, with atmospheric pressure onone side of the seal and about 10⁻¹⁰ Torr absolute pressure on the otherside of the seal. If the vacuum testing indicates no leakage, the sealis considered to be performing in a satisfactory manner.

Metal-comprised layer 202 preferably has a cross-sectional thickness,shown in FIG. 4 as “a”, of about 0.039 in. (1 mm) or less. The thicknessof metal-comprised layer 202 is important in determining the flexibilityof sealing means 140, and flexibility is critical to ability of sealingmeans 140 to perform. The composition of metal-comprised layer 202preferably exhibits a relatively low linear coefficient of thermalexpansion when alumina or aluminum nitride comprises upper platen 106.Preferably, the linear expansion coefficient ranges from about 2×10⁻³in./in./° C. to about 7×10⁻³ in./in./° C. at about 600° C.

Thin, metal comprising layer 202 can vary in height, shown in FIG. 4 as“b”, depending on the particular sealing apparatus application. For thepreferred embodiment of the sealing apparatus shown in FIG. 1, theheight of thin, metal-comprising layer 140 is typically about 0.195 in.(5 mm) or less. The length of the strip or ribbon of thin, metalcomprising-layer 202 depends on the sealing apparatus application, butmust extend completely along the length of the surfaces to be bridged asnecessary to provide a seal.

With reference to FIG. 2, brazing material 204 is applied along theedges of metal-comprising layer 202 for its entire length. Applicationof brazing material 204 is adjacent upper and lower exterior edges 206and 208, respectively, of metal-comprising layer 202 and is adjacent tothe complimentary surfaces of support platform housing 108 and upperplaten 106 which are to be in contact with metal-comprising layer 202via brazing material 204. Brazing is carried out in a brazing furnaceuntil such time that molecular intermixing is achieved between brazingmaterial 204 and the surface material of metal-comprising layer 202;between brazing material 204 and the surface of the material comprisingsupport platform housing 108 extension 142; and between brazing material204 and the surface of the material comprising upper platen 106. Thetime and temperature profile of the brazing material process depend onthe materials involved.

FIG. 3 shows an alternative embodiment of the sealing means, 300.Sealing means 300 is comprised of a thin, metal-comprising layer 302 inthe form of an accordion strip which is braised to upper platen 306 andplatform housing 108 extension 142 via brazing material 304. Thematerials of construction of metal-comprising layer 302 and brazingmaterial 304 are the same as those described with reference tometal-comprising layer 202 and brazing material 204 of FIG. 2. The useof an accordion-shaped strip as the metal-comprising layer 302 increasesthe flexibility of layer 302 so sealing means 300 can accommodate agreater difference in linear expansion coefficient between upper platen306 and support platform housing 308. Further, the amount of stressinduced within brazing material 304 is reduced by this alternativedesign.

The above-described preferred embodiments of the present invention arenot intended to limit the scope of the present invention as demonstratedby the claims which follow, as one skilled in the art can, with minimalexperimentation, extend the disclosed concepts of the invention to theclaimed scope of the invention.

What is claimed is:
 1. A method of avoiding or reducing the need toclean an upper surface of a semiconductor support platen, said methodcomprising: a) providing a removable insert which is located along anouter edge of said support platen and which provides an upper surfacewhich forms a portion of the upper surface of said support platen; andb) removing said removable insert when contaminant deposition materialsaccumulate on an upper, exposed surface of said insert to aconcentration which would necessitate cleaning of said upper surface ofsaid support platen.
 2. The method of claim 1, including an additionalstep: c) replacing said removable insert with a clean insert.
 3. Themethod of claim 2, wherein said clean insert is a recycled insert. 4.The method of claim 1, wherein said support platen is circular in shapeand said insert is a circular ring which is located at the peripheraledge of said support platen.